I had problems using pdftk with the cat parameter had a better success with output.
The following command worked for me:
pdftk file_1.pdf file_1.pdf file_1.pdf file_1.pdf cat output.pdf
Using cat produced the following error:
Error: Unexpected text in page range end, here:
output.pdf
Exiting.
Acceptable keywords, for example: "even" or "odd".
To rotate pages, use: "north" "south" "east"
"west" "left" "right" or "down"
Errors encountered. No output created.
Done. Input errors, so no output created.
http://www.pdflabs.com/docs/pdftk-cli-examples/.
I created a 172mb PDF is no time at all.